This invention relates to variable pitch propellers, and more particularly, to an improved bearing assembly that is located between the interface of a variable pitch propeller blade pin and a reciprocal actuator for changing the pitch of the blade.
Many conventional variable pitch propeller systems use a scotch yoke type mechanism to convert linear motion of a hydraulic piston to rotational motion which rotates propeller blades in a hub to change their pitch. Such a pitch change mechanism requires a dynamic interface beetween the propeller blades and the yoke and actuator assembly. This interface must be capable of accommodating relative motion between the yoke and the propeller blade pins as well as handling misalignment between the components.
To provide these capabilities, conventionally two types of bearings have been employed. One type of bearing is a self-aligning spherical bearing while the other is a crowned roller bearing. To control the wear rate between the bearings and the yoke and actuator assembly, the contact stress level between the bearings and the yoke must be kept within a specific range. This is accomplished by sizing the bearings accordingly. In the usual case, for a given contact stress level held constant for a given load, self-aligning spherical bearings have the advantage of accommodating a larger misalignment between the components than a similar sized crowned roller bearing.
In order to increase the performance of crowned roller bearings, it would be desirable to increase the radius of curvature of the crown, thereby reducing contact stress at the point of contact of the bearing with the yoke. However, when this is done, as the parts flex during operation, if the height of the planar surface of the yoke is less than the end to end dimension of the bearing, the outer race surface of a crowned roller bearing tends to move toward a side edge of the planar surface of the yoke, that is, roll partially off the yoke surface. Alternatively, if the height of the planar surface of the yoke is greater than the end to end dimension of the bearing, the planar surface of the yoke tends to move toward an end of the bearing, that is, partially roll off of the bearing surface. Either way this results in a reduction in the area of contact between the two components, thereby increasing contact stress. Consequently, heretofore, it has not been possible to increase the radius of curvature of the outer race surface of a crowned roller bearing sufficiently to achieve the same operational capabilities as similarly sized self-aligning spherical bearing.
At the same time, the cost of a self-aligning spherical bearing is significantly greater than that of a crowned roller bearing. Thus, there is a need for a bearing that has the economic advantages of a crowned roller bearing which at the same time has the misalignment accommodating capabilities of a self-aligning spherical bearing. The present invention is directed to meeting that need.
It is the principal object of the invention to provide a new and improved bearing that may be used with cost effectiveness without sacrificing the capability of good misalignment managing qualities for use in a variable pitch propeller assembly.
An exemplary embodiment contemplates such a bearing in a variable pitch propeller assembly that includes a hub rotatable about a hub axis, at least one propeller blade having an end journaled in the hub for rotation about a blade axis that is generally perpendicular to the hub axis, a blade pin on the end of each blade and offset from the respective blade axis, a reciprocal blade actuator in the hub engaging the blade pins and operable, upon reciprocation, to cause the blades to rotate about the blade axes. The invention specifically contemplates bearings interposed between the reciprocal blade actuator and having rotatable races journaled on the blade pins with race surfaces that are surfaces of revolution engaging the reciprocal blade actuator. The improvement specifically includes the fact that the race surfaces have a somewhat lesser diameter at one side than at their other side.
In a preferred embodiment of the invention, the races have a crowned surfaced with a radius of curvature. The center point of the radius of curvature of the crowned surfaces is offset to one side of a plane perpendicular to the blade axis and extending through the center of the race.
In a highly preferred embodiment the center point is offset to be located between the blade and the center of the race.
In a preferred embodiment, each of the races includes a central bore with a step adjacent the end of the race remote from the blade and a polymeric bushing is disposed within the bore and abuts the step. The bushing is disposed about the blade pin and a retainer is mounted to the blade pin for retaining the race and the bushing on the blade pin.
Preferably, the bushing is formed of a polymer having high strength and low creep properties.
In a preferred embodiment, the retainer engages the bushing and is spaced from the race and the bushing.
In a highly preferred embodiment of the invention, the retainer is a cap-shaped element have an axial flange for receiving an end of the blade pin and with the axial flange closely adjacent the end of the bushing adjacent the step.
In addition, it is even more preferred that the cup-shaped element has a radial flange that overlies but is separated from the remote end of the race.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.